Venting caps must be able to function properly in a wide span of end uses and storage and transportation conditions. For a wide group of consumer packages the following major prerequisites are required: (a) The caps must vent air at low pressure difference built-up. (b) They must not permit liquid exit even at high pressure built-up in the container. (c) They must retain these properties in the most extreme conditions of transportation and storage. (d) Their cost of production must be low and the materials and parts required for their manufacturing must be readily available. From the evaluation of vented cap technology available at present it was found that in all cases the vented caps proposed or offered in the market do not conform sufficiently to one or more of the above prerequisites.
The following arrangements have been tried to overcome this problem.
A first attempt was the creation of plastic bottles with very thick walls and specially design features to prevent deformation. Such bottles are expensive and environmentally unsuitable because of the need to use excessive plastic material (see for example Packaging Techn. & Sci., 6(1993),23-29).
A second attempt was the capping of the bottles with caps fitted with porous semipermeable membranes, which permit the passage of gases but not of liquids. The caps have suitable openings permitting the gas to exit to the environment. The major problem of this arrangement consists in the need of a much higher pressure difference to guarantee functionality when the membrane is wet. Such caps are described in the following patents and patent applications: EP-0 408 378 (W. L. Gore), WO 94/26614 (Procter & Gamble), WO 94/22553 (W. L. Gore), DE-2 341 414 (Hesser). There are two main problems related to such caps. One is the high cost of the semipermeable membrane used and the limited sources of their supply. The other and most important problem is that when the membranes come in contact with the liquid contents (which almost always happens when the packages are transported or stored in a tilted or horizontal position) there is a change in their permeation characteristics. Thus, instead of permitting the gases to flow at low pressure differences, the once moistened membranes require much higher pressure differences to permit gas flow. There are cases where a membrane is specified to permit gas flow at 5 mbar pressure difference which rises to 250 mbar when the membrane is wetted. To overcome this second problem, a protective cap of the membrane is proposed in EP-0 110 046 (Rhein-Conti) and in Greek patent application 960100443. Such attempts increase excessively the cost of caps.
A third attempt was the use of caps containing an outlet covered by an elastic membrane with a thin split which would permit the exit of gas above certain pressure but was impermeable to the liquid contents. Such caps are described in EP-0 555 623, GB-1 534 570, U.S. Pat. No. 5,143,236 (L'Oreal), U.S. Pat. No. 4,896,789 and Greek patent application 96011443. The drawback of such caps is the lack of complete selectivity in permitting the exit of gas but not of the liquid. Normally, one can see liquid bubbles coming out of such caps during storage. It has been found in our experiments that the size and shape of the slit, the geometry of the elastic membrane, and the characteristics of the elastic material of the membrane are so critical that even the slightest deviation creates this non-selectivity problem.
A fourth attempt uses caps containing an inside elastic sealing disc, seated on a ribbed or grooved non-flat surface on the underside of the cap. In theory a gas under pressure inside the bottle deforms the elastic disc and escapes through the openings created between the deformed disc and the non flat surface of the cap (U.S. Pat. No. 5,242,069 (Henkel), DE-3 611 089 (Henkel), WO 94/13549 (Wazel), EP-0 241 780 (Henkel), U.S. Pat. No. 5,457,943 (Hertramf)). The main drawback in such caps, in addition to their non-selectivity, is the fact that very high pressure differences are required to deform the disc (200 mbar or more). At such high pressures the plastic bottle is already deformed before the escape of gas.